In using explosives to dislodge or heave material such as in a quarry, it is quite common practice to drill a number of boreholes, charge the boreholes with explosive material such as ANFO or ammonium nitrate slurry, and then detonate the explosure material in the boreholes in sequence to produce the desired movement of material. Since material of the type used in boreholes generally requires an intermediate primer of high explosive material for detonation, various arrangements have been used for priming the boreholes for detonation.
A common arrangement is to secure a detonating cord through the normal opening in a primer and drop the primer and cord to the lower portion of the borehole. Thereafter, explosive material is placed into the borehole or the borehole is filled further with explosive material. In some instances, the borehole is provided with several sections of explosive material separated by non-explosive material, such as soil. In these instances, a primer is generally required for each of the separate explosive charges. To accomplish this, as each section of charged explosive material is deposited, a primer is dropped down along the detonating cord forming the down line. After several charges are in place and primed, the same detonating cord can be used to explode all the primers simultaneously. This simultaneously explodes each of the various explosive charges within the borehole to provide maximum heave of the material being moved. These concepts of charging and priming boreholes with standard, available primers are well known and extensively used in the field.
In some instances, maximum earth movement can be accomplished by exploding or detonating various boreholes at different intervals during a single detonation. To accomplish this, the trunk lines used to detonate several detonating cords of different boreholes are interconnected by time delay devices. Thus, one group of boreholes controlled by one trunk line can be detonated at a slightly different time than another group of boreholes connected to a separate trunk line. These time delay connections take a variety of forms. Most commonly, they involve a time delay cartridge which is generally cylindrical and has internal structure which delays the propagation of a detonation wave therethrough for a preselected time. These cartridges are often connected at opposite ends to a relatively short section of commercial detonating cord. Thus, to interconnect two trunk lines for different detonating times, one of the time delay detonating cord sections is secured to one trunk line and the other detonating cord section is secured to the other trunk line. During detonation of one trunk line, there is a time delay until detonation of the next trunk line. Also, there are one piece molded time delay couplings which can be connected between somewhat standard detonating cords to provide the same preselected time delay. These cartridges or couplings are well known in the art and can be timed for delays of approximately 5 milliseconds to upwardly of several seconds. Indeed, some time delays are rated at zero time delays and they are often used for a connection between a primer and a low energy type of detonating cord, such as a detonating cord having a grain loading of less than about 10 grains per linear foot. Also, such zero time delay devices can be used with low energy detonating cord of the type having a hollow tube with an inner cylindrical wall coated by explosive material or filled with a combustible gas. In all instances, the time delay devices provide a preselected time shift from the somewhat instantaneous detonation occurring in a detonating cord. The availability and use of these various time delay devices used with detonating cords are well known. In addition, some time delay devices may be used with electrical caps which can be used to explode the high explosive of a primer for detonating the charge in a borehole at a preselected time after an electrical signal.
In recent years, governmental regulations have been adopted which affect the use of explosives of the type described above. One of these regulations, which is becoming quite common, limits the amount of explosive material which can be detonated at any given time within a certain distance from an inhabited building or from a highway or public transportation artery. This regulation has caused certain modifications in the blasting techniques used in congested areas or in areas adjacent specific structures. Compliance with these regulations has resulted in the adoption of the concept of detonating the material in a borehole at different times to prevent a violation of regulations regarding the amount of explosives that can be detonated at any given time. The first attempt to provide a means of detonating several axially spaced explosive charges in a given borehole at different and distinct times has been the use of separate time delay electrical caps for detonating the primer in each of the different axially spaced explosive charges in a single borehole. This procedure involved the conversion of the detonating system into an electrical system. As is well known, there are certain environments in which an electrical system is not acceptable or completely satisfactory. For instance, when electrical equipment is being used in the vicinity or during electrical storms. When electrical lines are laid for a detonation, these lines can act as an antenna and can be actuated in some unusual situations by electromagnetic waves, such as radio waves. Also, many users are well accustomed to detonating cord and somewhat hesitant to replace such systems with electrical systems to comply with governmental regulations. Thus, there is a substantial amount of effort devoted to the modification of the detonating cord system into a system which will comply with regulations and provide sequential detonation of separate charges axially spaced within a single borehole. One of the most common systems is to provide a separate time delay cartridge in the detonating cord extending to each of several primers within the borehole. This requires the use of separate and distinct down lines extending to the different primers at axially spaced positions within the borehole. This type of arrangement is time consuming and costly. Another arrangement is to provide time delay cartridges at the primers themselves and use several low energy detonating cords extending from the upper trunk line to the separate primers within a given borehole. This concept is not substantially different from the concept of using time delay devices in the down line itself since separate and distinct down lines are required for each primer to produce the time delay required for sequential detonation of the axially spaced charges.
Prior to the present invention, an improved primer unit has been developed for exploding spaced charges in a borehole. This delay primer unit is disclosed in prior application Ser. No. 740,799, filed Nov. 11, 1976, which is incorporated by reference herein as background information. In accordance with the disclosure of this prior application, an explosive element, or primer, can be moved along a down line formed from a detonating cord and detonated at a time delay value after the down line by an interconnecting time delay element. Thus, each of these units can be positioned with axially spaced explosive charges to provide serially detonated explosive charges in a borehole. This prior concept utilizes a single detonating cord for detonating the various explosive charges in time series. The present invention relates to an improvement in this prior time delay primer unit which does not require the use of a cast primer or other preformed primer as shown in the preferred embodiment of the prior application.